ASSESSING PHYSICOCHEMICAL AND MICROBIAL PROPERTIES OF SOILS IN PERI‐URBAN AGRICULTURAL AREAS: SOUTH PUGET SOUND, WA

Prabesh Khanal

doi:10.7273/000007257

Urbanization disturbs the natural hydrologic regime by increasing impervious land area, leading to increased runoff peak and volume, urban flooding, and diffuse pollution. Such changes can adversely impact local ecosystems, including the soil system. This study aims to better understand how the impact of a changed hydrological regime due to urbanization affects the health of agricultural lands on the urban fringe, specifically soil physical, chemical, and microbial properties. As a first step, I evaluated soil and water properties at selected farms in two rapidly urbanizing watersheds in South Puget Sound, USA. Soil, surface‐, and groundwater at three farms, Farm 1 (Green River watershed, King County) and Farms 2 and 3 (Puyallup River watershed, Pierce County) in western Washington State, were sampled twice (fall 2022, spring 2023) and their physiochemical and microbial properties were analyzed. Several major soil physicochemical and microbial properties exhibited significant spatial (but not temporal) variability. For example, soil texture at Farm 1 differed significantly from that at Farms 2 and 3 (P at significance level α = 0.05 as in all statistical tests in this study). Total Kjeldhal Nitrogen (TKN), Total Nitrogen (TN), Total Phosphorous (TP), and organic matter (OM) at Farm 1 were significantly lower than at Farms 2 and 3. All analyzed properties except pH did not differ significantly between the two sampling events. Soil pH ranged 4.9–6.6 at Farm 3, showing acidic conditions at certain sampling locations, with the mean differing significantly from those at Farms 1 and 2. Acidic soil conditions can limit nutrient availability and increase toxic metal solubility, adversely impacting the growth environment for plants and microorganisms. TN, TP, and dissolved copper and zinc concentrations in the groundwater samples met WA groundwater quality standards. However, TP in the Green River water samples exceeded the US EPA limit for preventing eutrophication. Farm 3 had significantly lower microbial richness than those at Farms 1 and 2, suggesting that lower pH might, among other factors, contribute to reduced bacterial richness and diversity. There was a greater similarity in microbial properties between Farms 2 and 3 (than between Farms 1 and 2 or Farms 1 and 3), possibly because these two farms were located close to each other on the Puyallup River floodplain, with the same soil texture and subject to similar impacts of urban development. Both watershed characteristics and land‐use and management practices have likely contributed to the spatial variations in, and dynamics of, soil and water conditions at the three peri‐urban farms.

ASSESSING PHYSICOCHEMICAL AND MICROBIAL PROPERTIES OF SOILS IN PERI‐URBAN AGRICULTURAL AREAS: SOUTH PUGET SOUND, WA

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